The neural crest is a multipotent stem cell population that migrates long distances in vertebrate embryos and gives rise to a large number of essential cell types including neurons and glia of the peripheral nervous system, melanocytes, chromaffin cells of the adrenal gland, and most of the craniofacial skeleton. Neural crest cells undergo an epithelial to mesenchymal transition (EMT), and migrate from the dorsal neural tube and populate distant target sites where they give rise to these derivatives. The molecular mechanisms that direct the development and differentiation of neural crest cells remains poorly understood, however, a number of key transcriptional regulators of these processes have been identified. SoxE family transcription factors (Sox8, Sox9, and Sox10) are essential for several aspects of neural crest cell development, including the formation of the craniofacial skeleton. SoxE factors act reiteratively throughout neural crest development to promote the initial formation of neural crest cells, the maintenance of their stem cell state, and the differentiation of a subset of neural crest derivatives. One mechanism via which SoxE factors can be regulated is through the actions of a family of related Sox proteins, the SoxD proteins (Sox5/6/13). The SoxD family transcription factor Sox5 is expressed in early neural crest cells and in a subset of neural crest derivatives including skeletal tissue. Where it has been examined, Sox5 appears to function in part by modulating the activity of SoxE factors in a context dependant fashion; Sox5 can inhibit Sox10-dependent activation of melanocyte promoters but cooperates with Sox9 to drive transcription of chondrocyte target genes. I have found that Sox5 is expressed throughout many stages of neural crest development in Xenopus and that it plays essential roles in this important cell type. In this proposal I will build on these strong preliminary findings in order to better understand the role of Sox5 in regulating SoxE function during neural crest development. )